1. Field of the invention
This invention relates to a method of filtering x-rays using a freestanding thin film of a polymer, metal-polymer composite, mesh-supported polymer film, or mesh-supported metal-polymer composite. The filtering of x-rays in x-ray optics is done with the use of a "bandpass filter" that selects, or allows to pass, only a certain band of frequencies of the radiation that impinges on the filter, and in this invention the polymer is a polybenzimidazole (PBI) or a polyphenylquinoxaline (PPQ).
2. Prior Art
Freestanding, thin film composites of a metal and a polymer are utilized extensively in x-ray optics. In x-ray astronomy, such films were used to eliminate visible light from solar spectrographs nearly 30 years ago. Since that time they have served as bandpass filters for telescopes that operate in the extreme-ultraviolet and soft x-ray regions of the electromagnetic spectrum. A partial review of the materials that have been used in x-ray telescope filters was written by F. R. Powell, P. W. Vedder, J. F. Lindblom, and S. F. Powell in "Thin film filter performance for extreme ultraviolet and x-ray applications," Opt. Eng. 29, 614-624 (1990). An example of the use of filters in a specific telescope has been described by J. F. Lindblom, R. H. O'Neal, A. B. C. Walker, Jr., F. R. Powell, T. W. Barbee, Jr., R. B. Hoover, and S. F. Powell in "Multi-spectral solar telescope array IV: the soft x-ray and extreme ultraviolet filters," Opt. Eng. 30, 1134-1141 (1991).
Additionally, polymeric films have been used as entrance windows for proportional counter x-ray detectors. One example of their use in proportional counter windows was documented by M. Heppener and D. G. Simons in "A large aperture imaging gas scintillation proportional counter, " in X-Ray Instrumentation in Astronomy II, L. Golub, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 982, 139-146 (1988); a second example was given by U. G. Briel, E. Pfeffermann, G. Hartner, and G. Hasinger in "X-ray calibration of the ROSAT position sensitive proportional counter," in X-Ray Instrumentation in Astronomy II, L. Golub, ed., Proc. Soc. Photo-Opt. Instrum. Eng. 982, 401-408 (1988).
Metal-polymer composite filters have a similar function in x-ray microscopy, where they transmit the bandpass of interest and filter out the unwanted longer wavelength radiation. An example of the use of a carbon-phthalocyanine filter in an x-ray microscope was presented by R. B. Hoover, D. L. Shealy, B. R. Brinkley, P. C. Baker, T. W. Barbee, Jr., and A. B. C. Walker, Jr. in "Development of the water window imaging x-ray microscope utilizing normal-incidence multilayer optics," Opt. Eng. 30, 1086-1093 (1991). A discussion of a tin-Formvar filter was given by J. F. Seely in "Transmission filter for the `water-window` wavelength region 23-44 .ANG.," Opt. Commun. 70, 207-212 (1989).
Additional uses of metal-polymer composites in x-ray optics have included pressure windows in synchrotron radiation facilities and x-ray lithography masks. In the present document, use of the term "x-ray filter" shall be construed to include every application in which a membrane is inserted into the optical path of x-ray or extreme ultraviolet radiation, including pressure windows.
Many types of polymers have been evaluated for the above-mentionned applications. These materials have widely varying structures, and include: 1) poly(vinyl formal) (Formvar, Shawinigan Resins Corp. and Monsanto), 2) poly(p-xylylene) (Parylene N, Union Carbide Corp.), 3) copolymers of vinyl chloride and vinyl acetate (VYNS, Union Carbide Corp.), and 4) polycarbonate (Lexan, General Electric Co.). Data have been accumulated for the cellulosic family comprising 5) cellulose acetate (Zapon) and 6) cellulose nitrate (Parlodion, Collodion). Other choices have been 7), poly(ethylene terephthalate) (Mylar), 8) poly(methyl methacrylate), and 9) polypropylene.
Polyimides have been assessed as potential candidates for proportional counter entrance windows by S. Nenonen, H. Sipilai, P. Jalas, and R. Mutikainen, "Soft x-ray windows for position sensitive proportional counters," in EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy, H. S. Hudson and O. H. Siegmund, eds., Proc. Soc. Photo-Opt. Instrum. Eng. 1344, 100-105 (1990). The thermal stability of an aromatic polymide distinguishes this class of polymers from its counterparts listed above. Polyimides are believed to possess superior mechanical properties as well, and for these reasons polyimides are being evaluated as the polymeric component of a composite bandpass filter.
A partial review of the polymers previously used in metal-polymer composites was given by S. F. Powellb M. J. Allen, and T. D. Willis in "Tin-polyimide and Indium-polyimide thin film composites as soft x-ray bandpass filters," AppL Opt. 32, 4855-4859 (1993).
A discussion of the rigidity (and the opposite of rigidity, which is flexibility) of a polyimide has been covered by C. P. Wong in a chapter entitled "Recent Advances in IC Passivation and Encapsulation: Process Techniques" in Polymers for Electronic and Photonic Applications, C. P. Wong, ed. (Academic Press, New York, 1993), p. 202. Polyimides derived from the dianhydride monomer pyromellitic dianhydride (PMDA) or 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BTCA) are rigid, while those synthesized from bis(3,4-dicarboxyphenyl) ether dianhydride (ODPA) are flexible. Polyimides constructed from benzidine or the diamine para-phenylenediamine (PPD) will be rigid, whereas those derived from 4,4'-diaminodiphenyl ether (ODA, or oxydianiline) will be flexible. ODPA and ODA are flexible because of the in-chain rotational movement allowed around the oxygen atom of the ether linkage. Additionally, these flexible polymers will by chemical definition contain oxygen because of the ether linkage. Rigid polymers are more desirable for x-ray filter and pressure window applications because of their superior strength (which in a thin film is most appropriately described by the film's biaxial modulus) and resistance to elongation.
There is no mention in the literature of a polymeric x-ray filter or pressure window fabricated from a polymer with a rigid configuration, similar to the polyimide BTCA-PPD, which is also oxygen-free. The importance of an oxygen-free "analog" or "variant" of the BTCA-PPD polyimide polymer will be discussed shortly.
The mechanical properties of an x-ray filter consisting of a thin film of a polymer or a metal-polymer composite may be enhanced by laminating the freestanding polymer film or metal-polymer composite to a wire screen or mesh. The use of a mesh supported, 1000 angstrom thick film of carbon as the entrance window for an x-ray microscope's proportional counter is discussed in Optical Systems for Soft X-Rays by A. G. Mchette, p. 261, Plenum Publishing Corporation (1986). Further discussion of the use of mesh to strengthen an x-ray filter is discussed in U.S. Pat. No. 5,261,977, entitled "Method of Fabricating Free-Standing Thin Films of Polyimide (issued Nov. 16, 1993). This patent is herein incorporated by reference.
"Freestanding," in this context, means that the film is mounted on a frame. A schematic diagram of a freestanding film has been shown in FIG. 6 of U.S. Pat. No. 5,261,977. A freestanding film is supported by the frame only at the film's edges, thus creating a central region of film that is not in direct contact with the frame. The central region of the film, in fact, is not in contact with any surface. "Freestanding" also refers to the fact that the film has been removed from the substrate on which the film had been cast, formed, or deposited. An appropriate thickness of the polymer film or metal-polymer composite, when it is freestanding and frame-supported, is five microns or less.